1. Field of the Invention
The present invention relates to an active vibration damping device for use as an automotive engine mount, body mount, or the like in order to produce active or countervailing damping action of vibration to be damped, and relates in particular to an active vibration damping device adapted to provide active vibration damping action by means of employing an oscillation member to constitute part of the wall of a pressure receiving chamber with a non-compressible fluid sealed therein, and controlling pressure within the pressure receiving chamber by means of exciting actuation of the oscillation member with a solenoid type actuator.
2. Description of the Related Art
Fluid filled type active vibration damping devices are known as one type of vibration damping device used as a vibration damping connector or vibration damping support interposed between components making up a vibration transmission system. Such a vibration damping device typically comprises a pressure receiving chamber a portion of whose wall is composed of a main rubber elastic body linking a first mounting member and a second mounting member; and an oscillation member making up part of the wall of the pressure receiving chamber, and actuated from the outside under the control of an actuator. Such devices are taught in JP-A-9-49541 and JP-A-2000-283214, for example. In this kind of active vibration damping device, pressure within the pressure receiving chamber is regulated according to the input vibration to be damped, so as to be able to cancel out the input vibration to provide active vibration damping action.
In fluid filled type active vibration damping devices of this kind, in order to achieve effective damping action, it is important to control pressure fluctuations within the pressure receiving chamber to frequency and phase corresponding with high accuracy to input vibration.
The actuator used to apply actuating force to the oscillation member is favorably a solenoid type actuator, as disclosed in JP-A-9-49541 and JP-A-2000-283214 above. Such an actuator typically has a structure wherein a movable member is positioned displaceably inserted in a stator having a yoke member is attached about a coil to form a stator-side magnetic path, and current is passed through the coil to create actuating force in the axial direction between the stator and the movable element.
In the event that a solenoid actuator of this kind is to be employed, for example, in an automotive engine mount or other vibration damping device, there is a problem in that the actuator, and hence the vibration damping device itself, may not readily afford satisfactory durability and reliability with regard to operating performance. This is due to the fact that, while in the case of an automotive engine mount, it is necessary for the device to be able to provide continuous vibration damping for a predetermined time period in a high frequency range of several tens of Hz and above for an extended time of several years or more, a typical solenoid actuator cannot consistently maintain such continuous operation in a high frequency range for an extended period.
With the foregoing in view, it has been contemplated to attach a guide sleeve to the stator to form a guide hole with good sliding on the inside peripheral face of the guide sleeve, so as to provide a guide mechanism that will lessen the damage in the event that the movable element should come into contact with the inside face of the guide hole.
However, since the interval between the movable element and the guide hole consists of a very small gap all the way around, it is difficult to avoid the movable element coming into contact with the inside face of the guide hole, due to error in component tolerances and assembly, or change of parts over time. Particularly in vibration damping devices such as those discussed above, even if the actuator by itself is a high precision component, it is nevertheless extremely difficult to maintain a high degree of accuracy of relative position of the stator and the movable element while installed in the vibration damping device, so contact of the movable element with the inside face of the guide hole is unavoidable in actual practice.
The reason for this lies in the fact that vibration damping devices typically employ rubber elastic elements for displaceably supporting the oscillation member, but since rubber elastic elements experience molding shrinkage, it is not possible to control their dimensions with high accuracy as with metal fittings. Thus, in a vibration damping device, it is not possible to avoid positional deviation in the axis-perpendicular direction of the stator affixed to the second mounting member and the movable element attached to the oscillation member, and contact between the movable element and the stator due to positional deviation is inevitable.
In most instances, contact between the movable element and the stator in the axis-perpendicular direction occurs with the upper end or lower end of the movable element as the point of contact with the stator. However, when the movable element undergoes relative displacement and inclines in the twisting direction with respect to the stator, a first axial end and the other end of the movable element mutually come into proximity with the stator on opposite sides in the axis-perpendicular direction, as a result of which magnetic force acting on both ends serves to produce further inclination of the stator.
Such inclination of the stator in many instances results in contact of the movable element and the stator occurring as point contact at both axial ends. As a result, there is appreciable contact plane pressure in the areas of contact of the movable element with the stator, so that wear is accelerated in the contact areas, and the problems of insufficient durability and inconsistent operating characteristics tend to occur.
Additionally, it is typical to subject the contact areas of the movable element and the stator to slide coating with fluororesin in order to improve slide, to corrosion resistant coating by means of plating process, or to various other coatings. At contact points resulting from inclination of the movable element as discussed above, such coatings tend to peel due to scratching, thus exacerbating the problems of poor durability and operational stability.
To address such problems, JP-A-2002-25820 proposes to furnish a bearing composed of a number of rigid spheres and an elastic member, to provide a structure for controlling bias of the movable element. However, this bearing has a complicated structure and numerous parts, and thus would be difficult to employ in terms of the production process and production costs. Additionally, the dimensional accuracy of the bearing per se and the assembly accuracy when attaching the bearing to the actuator would tend to be problematic. Thus, even if such a bearing were employed, it would not necessarily translate to effective results in terms of durability and operational stability.